$ composer init --require=drush/drush:6.* -n
change your version if you need a new version like 7. See more instruction at the drush doc site.

# Configure the path Composer should use for the Drush vendor binaries.

$ composer config bin-dir /usr/local/bin

# Install Drush.
$ composer install

Now test you drush by typing ‘drush’

$ drush

If you have done everything right you should get drush commands shown below.

$ drush
Execute a drush command. Run `drush help [command]` to view command-specific
help. Run `drush topic` to read even more documentation.

Global options (see `drush topic core-global-options` for the full list):
-d, --debug Display even more information,
including internal messages.
-h, --help This help system.
-n, --no Assume 'no' as answer to all prompts.
-r , --root= Drupal root directory to use
(default: current directory).
-s, --simulate Simulate all relevant actions (don't
actually change the system).
-l , URI of the drupal site to use (only
--uri= needed in multisite environments or
when running on an alternate port).
-v, --verbose Display extra information about the
command.
-y, --yes Assume 'yes' as answer to all
prompts.